A typical image displayed on a monitor or display screen that uses a cathode ray tube to generate the image, is made up of a plurality of horizontal lines that are created to define the image. In order to synchronize the line generation and creation of subsequent pictures, synchronization (sync) signals are used. Typically a horizontal and a vertical sync signal (H-sync, and V-sync, respectively), are defined, as illustrated in FIG. 1, in which a positive polarity signal 100 and a negative polarity signal 120 are shown.
Several approaches have been used in the past to detect the polarity of H-sync and V-sync. One approach is to make use of a low pass filter, which, in its simplest form, comprises a resistor and a capacitor to ground. The capacitor will effectively charge up during the positive portion of the cycle and discharge during the negative portion of the cycle. Thus, in the case of a positive polarity signal (signal 100 in FIG. 1) in which the positive portion is less than the negative portion, the capacitor will not be charged to a “high” level. while a negative polarity signal (signal 102 in FIG. 1) will charge up to a “high” level. However, the frequency range of these signals is quite low (typically in the kHz range for H-sync, and in the Hz range for V-sync). Thus, the capacitor used in such a filter approach has to be quite large (of the order of μF for H-sync). Due to the size of the capacitor that would be needed, it is unrealistic to integrate the capacitor structure on a semiconductor chip. Accordingly, an external capacitor has to be used, with a chip pin to the external capacitor. The need for discrete devices in implementing the solution therefore increases the cost, implementation effort and size.
Another approach has been to make use of a digital implementation that avoids the need for a capacitor. However this typically relies on a system clock, thus requiring one or more pins on the chip for connecting to an external clock or crystal. Thus, again external components are required.
The present invention proposes a method of detecting the polarity of sync signal that avoids the need for external components.